Quick Tip: Building a Cold Frame

Build a cold frame to protect your less established plantings in early spring and fall.

Gardening doesn’t have to be just a summertime activity, even in colder climates. Spend a weekend building a cold frame this spring and you can get started earlier and grow later into the fall.

What Are Cold Frames?Cold frames have been used for centuries to start cold-tolerant plants in the early spring, harden off seedlings before transplanting, shelter tender perennials and even overwinter plants and cuttings.

A cold frame is just a box set over the garden with a transparent roof to trap the sun’s heat and the earth’s moisture. Hot beds go one step further with electric cables or a bed of manure just below grade to heat things up. But in most climates, a cold frame’s 5- to 10-degree difference and protection from wind and frost is all you need to start your garden weeks earlier.

Cold Frame Dimensions and MaterialsThe ideal size for a cold frame is 3 by at least 6 feet, about 12 inches deep along the front sloping to 18 inches at the back. You can use 1×12-inch pressure-treated lumber nailed or screwed or try prefabricated corners for easy dismantling and storage. For the cover, old windows work great, but a panel of Plexiglas, fiberglass or even a double layer of plastic sheeting on a frame will also work. Even easier, check out your favorite gardening catalog for a ready-made model.

Cold Frame Orientation and Ideal TemperaturesOrient your cold frame toward the south or southwest, near the house so you can water it easily and monitor the temperature inside. Spring and fall plants do best at about 60 degrees; summer plants at 75 degrees or below. Plants will wither if they get too hot, though, so once the outdoor temperature goes above 40 degrees, prop the lid open about 6 inches. When the temperature reaches a consistent 50 degrees or above, you can remove the lid altogether during the sunny hours and replace it on those chilly spring nights.